1. Field of the Invention
This invention relates to intracavity doubled lasers and more particularly to intracavity doubled lasers that are amplitude stable.
2. The Prior Art
There are many applications of visible lasers that require low amplitude noise, high efficiency, long operating life time, and compact size. Gas lasers can supply visible light with output power in the range of many watts with low amplitude noise but they are very inefficient, typically operating at electrical efficiencies of less then 0.1%. Gas lasers are also usually quite large and have lifetimes of only a few thousand hours, limited by the operating life of the plasma tube. There are many applications that would benefit from availability of a low-noise, efficient, long-lived solid-state laser capable of operating in the visible region of the optical spectrum.
One known prior-art method for obtaining visible emission from a solid-state laser is to insert a nonlinear doubling crystal into the cavity of an infrared laser. This method allows compact, and efficient laser designs with output powers from milliwatts to many watts in the blue, green, and red regions of the optical spectrum.
Previous designs of intracavity doubled solid-state lasers exhibit amplitude fluctuations in the visible output beam caused by nonlinear processes occurring in the intracavity nonlinear crystal. See, for example, T. Baer, J. Opt. Soc. Am. B, Vol. 3, No. 9, pp. 1175-1180 (1976). These amplitude fluctuations are often large in magnitude, often approaching 100% modulation, and have greatly limited the number of applications that could utilize these laser systems.
The amplitude fluctuations in prior-art intracavity doubled solid-state lasers have been attributed to a nonlinear coupling of the intensities of the different longitudinal modes due to the process of sum generation taking place in the nonlinear crystal. The sum generation process causes the intensities of the longitudinal modes to be interdependent since the intracavity losses experienced by each mode are determined by the intensities of the other modes. At steady state, the gain experienced by each mode is equal to the intracavity losses. However, if one mode increases in intensity, it can extinguish the other modes by increasing their losses, causing the other modes to drop below the lasing threshold. In this manner, sum generation destabilizes the laser system and results in chaotic pulsation of the laser output.
Several methods have been demonstrated that reduce these amplitude instabilities. One method, disclosed in U.S. Pat. Nos. 4,656,635 and 4,701,929 to Baer, is to run the laser in a single longitudinal mode by adding additional intracavity elements such as an etalon, by making the laser cavity very short (see U.S. Pat. No. 5,430,754 to Suzuki), or by designing the laser in a ring configuration so that only one mode oscillates (U.S. Pat. No. 5,052,815 to Nightengale et al.).
Another method utilized in the prior art is to add a quarter wave plate to the laser cavity which forces the laser to operate in orthogonally polarized modes as shown in Oka et al., Opt. Lett 13, 805 (1988). This reduces the sum generation process and stabilizes the laser output. This method has the disadvantage that it only works with specific nonlinear crystals (type two phase matching) and it requires additional intracavity elements.
All of these noise reduction techniques typically require additional intracavity elements that complicate the laser design and often lower the efficiency of the laser and reduce the output power. Additionally, these methods often increase the laser system sensitivity to changes in the laser resonator temperature.
Another method for reducing the amplitude noise utilizes a long laser cavity to introduce many longitudinal modes, typically 50 to 100. See U.S. Pat. No. 5,446,749 to Nighan et al. These many longitudinal modes provide an averaging effect which reduces the total a mount of laser noise. The individual modes are still unstable but the large number of modes reduces the magnitude of this instability. In some applications this residual instability of the individual modes may be unacceptable. This design in general requires a long laser resonator (greater than about 1 m in length) which is unsuitable for many applications where small designs are more desirable.
There is a need for a compact, efficient, intracavity doubled laser that can operate in several longitudinal modes with improved amplitude stability and that does not require any additional intracavity elements.
It is therefore an object of the present invention to provide an intracavity doubled laser that is amplitude stable and that runs in several longitudinal modes.
Yet another object of the present invention is to provide an intracavity doubled laser that is amplitude stable and is very efficient.
Another object of the present invention to provide an amplitude stable system that requires no additional optical elements.
A further object of the present invention is provide an amplitude stable laser system that is compact.
Another object of the present invention is to provide an intracavity doubled, multi-longitudinal mode laser that is amplitude stable and that produces blue, green, or red light.